Miller effect-based circuit for splitting poles

ABSTRACT

A circuit for splitting poles between a first stage and a second inverting voltage-amplifier stage of an electronic circuit, comprises, in series between the output of the first stage and the output of the second stage, and in that order, a first capacitor, a second capacitor and a resistor. The circuit further comprises a voltage-divider bridge which is connected between a terminal delivering a substantially constant voltage and the output of the first stage. The output of the voltage-divider bridge is linked to the common node between the first capacitor and the second capacitor, in such a way that a first resistor of the voltage-divider bridge is connected in parallel with the first capacitor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a circuit for splitting polesbetween a first stage and a second stage of an electronic circuit,relying on the Miller effect.

[0003] It relates to the field of circuit design for components orelectronic circuits, especially monolithic integrated circuits, in CMOSor other technology. It finds applications, in particular, in amplifiercircuits.

[0004] 2. Description of the Related Art

[0005]FIG. 1 is the diagram of an electronic circuit, for example anamplifier, comprising a first stage 10, a second stage 20 and apole-splitting circuit 30 according to the prior art. The first stagebehaves like a transconductance which delivers or draws a currentthrough its output impedance. The second stage 20 is an invertingvoltage amplifier. The output signal from this stage is therefore inphase opposition (180°) with respect to the signal at the input.

[0006] In this example, the second stage 20 is arranged in series with,and downstream of, the first stage 10. The pole-splitting circuit 30 isarranged between the output S1 of the first stage 10 and the output S2of the second stage 20. It consists of the branch between the nodes S1and S2 which comprises, in series, a capacitor Cc and a resistor Rz ofrelatively high value. This branch forms a feedback loop from the outputS2 onto the output S1. The capacitance of this branch, which herecorresponds to the capacitor Cc, is called loop capacitance. In thisexample, moreover, the first stage 10 is an operational amplifier andthe second stage 20 is a power-output stage The latter consists of apower MOS transistor 21 in common-source mode, in series with a currentsource 22 between a power supply terminal delivering a positive powersupply voltage Vcc, on the one hand, and the ground on the other hand.The transistor 21 here is an N-type MOS transistor, the output S2 of thestage 20 being taken on the drain of this transistor and the currentsource 22 being arranged between Vcc and this drain. Such a stage istherefore an inverting voltage amplifier.

[0007] The pole-splitting circuit 30 provides feedback and has thefunction of separating the respective poles of the first and of thesecond stage, so as to facilitate control of the stability of thefeedback-type amplifier. More precisely, it makes it possible to shift,towards the low frequencies, the dominant pole p1 at the output S1 ofthe first stage 10. This is because, if the load resistance and the loadcapacitance at the output of the first stage 10 and at the output of thesecond stage 20 respectively are denoted Rout1 and Cout1 and Rout2 andCout2 respectively, then:

[0008] the dominant pole p1 at the output S1 of the first stage 10 isgiven by: $\begin{matrix}{{p1} = \frac{- 1}{{Rout1} \times {Cc} \times {Av2}}} & (1)\end{matrix}$

[0009] a second pole p2 is given by: $\begin{matrix}{{p2} = \frac{- 1}{{Rout1} \times \frac{Cout2}{Av2}}} & (2)\end{matrix}$

[0010] a third pole p3 is given by: $\begin{matrix}{{p3} = \frac{1}{{Rz} \times {Cout2}}} & (3)\end{matrix}$

[0011] and a zero z1 is given by: $\begin{matrix}{{z1} = \frac{1}{\left( {\frac{1}{gm2} - {Rz}} \right) \times {Cc}}} & (4)\end{matrix}$

[0012] wherein, further, Av2 is the voltage gain of the second stage 20and gm2 is the transconductance of the transistor 21 of the second stage20.

[0013] According to the principle known as the Miller effect, the loopcapacitance Cc is multiplied by the gain Av2 in expression (1) above.Stated otherwise, the capacitance Cc, which intervenes in the expressionof the dominant pole p1 at the output S1 of the first stage, is seen, onthis node, as being multiplied by the value Av2 of the gain of thesecond stage 20. This amounts to shifting the pole p1 towards the lowfrequencies.

[0014] In certain applications, the loop capacitance Cc has to withstandhigh potential differences. Thus, in the example represented in FIG. 1,in which the value of the signal Vout at the output S2 of the secondstage 10 can vary between 0 V (volts) and Vcc, the voltage at theterminals of Cc can reach a maximum of Vcc−Vt, where Vt is the thresholdvoltage of a MOS transistor (typically 0.7 V). It results therefrom thatthe voltage at the terminals of Cc can exceed 10 V as soon as Vcc ishigher than 10 V. However, the maximum value of the voltage which acapacitor, produced according to HF7CMOS technology, for example, canwithstand is substantially equal to 10 V.

[0015] The pole-splitting circuit according to the prior art istherefore not suitable for this type of application.

[0016] Consequently, a circuit structure for splitting poles which issuitable in applications where the voltage at the terminals of thecapacitor Cc can exceed the maximum voltage imposed by the technologyused for fabrication would be desirable.

BRIEF SUMMARY OF THE INVENTION

[0017] Aspects of the invention include a circuit for splitting polesbetween a first stage and a second inverting voltage-amplifier stage ofan electronic circuit. The circuit comprises, on the one hand, in seriesbetween the output of the first stage and the output of the secondstage, and in that order, a first capacitor, a second capacitor and aresistor, and, on the other hand, a voltage-divider bridge. Thevoltage-divider bridge is connected between a terminal delivering asubstantially constant voltage and the output of the first stage. Theoutput of the voltage-divider bridge is linked to the common nodebetween the first capacitor and the second capacitor, in such a way thata first resistor of the voltage-divider bridge is connected in parallelwith the first capacitor.

[0018] The fact of replacing the single loop capacitor Cc of FIG. 1 withtwo capacitors in series, and of imposing a defined potential at thecommon point between these capacitors by virtue of the voltage-dividerbridge, makes it possible to reduce the maximum voltage which might beapplied on each of these capacitors. The connection of the abovevoltage-divider bridge does not affect the feedback, even at lowfrequencies, whereby the Miller effect is maintained.

[0019] Another aspect of the invention relates to an electronic circuitcomprising a first stage and a second stage the respective outputs ofwhich are linked by a circuit for splitting poles as defined above.

BRIEF DESCRIPTION OF THE OF THE SEVERAL VIEWS OF THE DRAWINGS

[0020]FIG. 1, already analyzed, is the diagram of a pole-splittingcircuit according to the prior art;

[0021]FIG. 2 is the diagram of a pole-splitting circuit according to anembodiment of the invention; and

[0022]FIG. 3 is the diagram of an example electronic circuit comprisinga pole-splitting circuit according to FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

[0023]FIG. 2, in which the same elements as in FIG. 1 bear the samereferences, depicts a pole-splitting circuit 40 according to anembodiment of the present invention.

[0024] Circuit 40 comprises, in series between the output S1 of thefirst stage 10 and the output S2 of the second stage 20, and in thatorder, a capacitor Cc1, a capacitor Cc2 and a resistor Rz. The secondstage 20 is an inverting voltage-amplifier. The first stage 10 may be ofany type. In the example represented, the first stage 10 is also anamplifier.

[0025] Circuit 40 further comprises a voltage-divider bridge. The latteris connected between a terminal delivering a substantially constantvoltage (for example the positive power-supply terminal, delivering apositive power-supply voltage Vcc), and the output S1 of the first stage10. It comprises, in series between S1 and Vcc, and in that order, aresistor R1 and a resistor R2. The output S3 of the voltage-dividerbridge, i.e. the node which is common between resistors R1 and R2, islinked to the node B which is common between capacitors Cc1 and Cc2.Stated otherwise, the resistor R1 and the capacitor Cc1 are mounted inparallel between the output S1 of stage 10 and the node B.

[0026] In order to describe the functioning of the circuit 40, it isappropriate to distinguish the functioning at low frequencies on the onehand and the functioning at high frequencies on the other hand.

[0027] At low frequencies, the impedance of the capacitor Cc1 is verymuch greater than that of the resistor R1 of the divider bridge.Everything therefore happens as if the capacitor were replaced by anopen circuit. Consequently, the loop capacitance is determined by thecapacitor Cc2 alone. Nevertheless, by reason of the presence of thedivider bridge, the current absorbed or restored by the capacitor Cc2which, respectively, originates from or is delivered to the output S1 ofthe first stage 10, is less, by a ratio R2/(R1+R2) than thatrespectively absorbed or restored under the same conditions by the loopcapacitance Cc of the circuit 30 of FIG. 1. This is why, in order forthe pole p1 to keep the same value as that obtained with the circuit ofthe prior art represented in FIG. 1 (which value is given by therelationship (1) above), it is appropriate to choose the values of R1,R2 and Cc2 in such a way that: $\begin{matrix}{{Cc2} = {\frac{{R1} + {R2}}{R2} \times {Cc}}} & (5)\end{matrix}$

[0028] At high frequencies, the impedance of the capacitor Cc1 is verymuch less than that of the resistor R1. The voltage-divider bridge R1,R2 is therefore short-circuited by the capacitor Cc1. Consequently, theloop capacitance is determined by the capacitors Cc1 and Cc2 in series.This is why, in order for the pole p1 to keep the value obtained withthe circuit 30 of FIG. 1, it is appropriate to choose the values of Cc1and Cc2 in such a way that: $\begin{matrix}{{\frac{1}{Cc1} + \frac{1}{Cc2}} = \frac{1}{Cc}} & (6)\end{matrix}$

[0029] that is to say in such a way that: $\begin{matrix}{\frac{{Cc1} \times {Cc2}}{{Cc1} + {Cc2}} = {Cc}} & (7)\end{matrix}$

[0030] It results from the foregoing that the structure of the circuit40 makes it possible to obtain the separating of the poles relying onthe Miller effect. It will be noted that this would not be the case ifthe voltage-divider bridge R1, R2 were connected between thepower-supply terminal Vcc and the ground, instead of being connectedbetween the power-supply terminal Vcc and the output S1 of the firststage 10. In this case, at low frequencies, the current respectivelyabsorbed or restored by the capacitor Cc2 would originate from thepower-supply terminal Vcc or would be discharged to ground instead,respectively, of originating from, or of being delivered to the outputS1 of the first stage 10.

[0031] By combining relationships (5) and (7) above, there is obtained:

R2(Cc1+Cc2)=(R1+R2)×Cc1  (8)

[0032] Expression (8) above is simplified when, as is conventional, aproportionality ratio exists between R1 and R2. The values of R1 and R2are preferably chosen, moreover, in such a way that the total resistanceR1+R2 of the voltage-divider bridge is sufficiently high to limit thestatic current consumption in the bridge to a few hundreds ofnano-amperes (nA) at most.

[0033] The determining of the values of the components constituting thecircuit 40 can therefore be carried out in the following way:

[0034] a) the value of the loop capacitance Cc is determined, on thebasis of the desired value of the dominant pole p1 at the output S1 ofthe first stage 10, of the load resistance Rout1 of the first stage 10and of the gain Av2 of the second stage 20, by using relationship (1)above. This is carried out in the same way as for a circuit according tothe prior art such as the circuit 30 represented in FIG. 1;

[0035] b) next, values of R1, R2, Cc1 and Cc2 are determined whichsatisfy relationships (6) and (8) above, and which, moreover, make itpossible to obtain the desired limitation of the static current in thevoltage-divider bridge R1, R2.

[0036] Hence, in an example wherein the power-supply voltage Vcc isequal to+15 V, and wherein, at stage a) above, it is determined that theloop capacitance Cc must be equal to 2 pF (picofarads), the followingvalues may be chosen at stage b):

[0037] Cc1=6 pF;

[0038] Cc2=3 pF;

[0039] R1=10 MΩ (megohms); and,

[0040] R2=20 MΩ.

[0041] With the values of R1 and R2 above, the total resistance of thedivider bridge R1+R2 is equal to 30 MΩ. The maximum current in thevoltage-divider bridge, which is given by Vcc/(R1+R2), is then equal to500 nA. This value is relatively low, and in any event acceptable in themajority of applications.

[0042] It will be noted that the value of the poles p2 and p3 and of thezero z1 at the output S1 of the first stage 10, which are givenrespectively by relationships (2) to (4) above, are not altered by thesubstitution of a circuit 40 according to the depicted embodiment of theinvention for the circuit 30 according to the prior art.

[0043]FIG. 3, in which the same elements as in FIGS. 1 and 2 bear thesame references, is the diagram of an example of application of apole-splitting circuit according to the depicted embodiment of theinvention, in a defined electronic circuit 100. The latter is, forexample, a monolithic integrated circuit, produced according to theHF7CMOS technology. It comprises a first stage 10 which here consists ofan operational amplifier, a second stage 20 which here consists of apower-output amplifier, and a pole-splitting circuit 40 the structureand the functioning of which have been described in detail above, inconnection with the diagram of FIG. 2.

[0044] The capacitors Cc1 and Cc2 of the circuit 40 are capacitors knownas “poly 1/poly 2” capacitors. The resistors R1 and R2 of thevoltage-divider bridge of the circuit 40 are formed by the P-type MOStransistors mounted as diodes. In one example, these take the form ofMOS transistors with long channels in order to obtain a high resistancevalue (10 MΩ in the example). It is recalled that, indeed, theconduction resistance of an MOS transistor is inversely proportional tothe ratio W/L, where W and L are respectively the width and the lengthof the channel of the transistor. This embodiment is advantageous sincea long-channel MOS transistor takes up less space on the siliconsubstrate than an integrated resistor of 10 MΩ. In the example, R1consists of a single such transistor, whereas R2 consists of two suchtransistors in series, all these transistors being identical. In thiscase, R2=2×R1. Stated otherwise, the proportionality ratio of theresistors R1 and R2 with the total resistance R1+R2 of the dividerbridge is equal to one-third and two-thirds, respectively.

[0045] Relationship (8) above is then expressed in the form:

Cc1=2×Cc2  (9)

[0046] which facilitates the determining of the values of Cc1 and Cc2 onthe basis of the value of Cc and of relationships (6) and (9).

[0047] The fact that the transistors mounted as diodes, which constitutethe voltage-divider bridge, are all identical further facilitates theproduction on silicon.

[0048] When, as is the case for an operational amplifier as representedin FIG. 3, the first stage 10 of the electronic circuit features adifferential structure, it may be necessary to provide supplementarymeasures. This is because, with the voltage-divider bridge R1, R2 beinglinked to the output S1 of this stage, it applies an offset voltage ontothis output. This offset voltage creates an imbalance between theoutputs of the differential pair of the operational amplifier 10, one ofwhich corresponds to the output S1 of the operational amplifier 10 andthe other of which is a node S1′ which is not used as an output in thisexample. In order to compensate for the offset voltage on the outputS1′, the electronic circuit may comprise a compensation impedance R3linked to the node S1′ in such a way that this node, in static mode,exhibits a potential substantially equal to the static potential of theoutput S1 of the stage 10. This impedance R3 is, for example, a resistorwhich is linked between the power-supply terminal Vcc and the node S1′,and which exhibits a value substantially equal to the total resistancevalue R1+R2 of the voltage-divider bridge R1, R2 of the circuit 40.

[0049] In the embodiment example of FIG. 3, the compensation impedanceR3 advantageously consists of three MOS transistors connected as diodesand mounted in series, which are identical to those constituting thedivider bridge R1, R2.

[0050] The embodiments of the invention have been described above in anon-limiting application example. It will be noted that it applies toany type of electronic circuit, integrated or otherwise, whatever thetechnology used (CMOS, bi-CMOS, bipolar, etc.). Furthermore, the firststage is not necessarily an amplifier, and the first and the secondstage are not necessarily linked in series with one another, since theremay be another stage between the two of them.

I claim:
 1. A circuit for splitting poles between a first stage and aninverting voltage-amplifier second stage of an electronic circuit,comprising: connected between an output of the first stage and an outputof the second stage, and in order, a first capacitor, a second capacitorand a resistor; and a voltage-divider bridge connected between aterminal delivering a substantially constant voltage and the output ofthe first stage, the voltage-divider bridge having an output that islinked to a common node between the first capacitor and the secondcapacitor, in such a way that a first resistor of the voltage-dividerbridge is connected in parallel with the first capacitor.
 2. The circuitaccording to claim 1, wherein, where Cc1 and Cc2 are values of the firstcapacitor and of second capacitor, respectively, and where R1 and R2 arevalues of the first resistor and of a second resistor respectivelyconstituting the voltage-divider bridge, the following relationship issatisfied: R2×(Cc1+Cc2)=(R1+R2)×Cc1  (8)
 3. The circuit according toclaim 1 wherein a total resistance value of the voltage-divider bridgeis sufficiently high to limit the current consumption in thevoltage-divider bridge to a maximum of a few hundreds of nano-amperes.4. An electronic circuit comprising a first stage and a second invertingvoltage-amplifier stage having respective outputs that are linked by acircuit for splitting poles, the circuit for splitting poles comprising:connected between an output of the first stage and an output of thesecond stage, and in order, a first capacitor, a second capacitor and aresistor; and a voltage-divider bridge connected between a terminaldelivering a substantially constant voltage and the output of the firststage, the voltage-divider bridge having an output that is linked to acommon node between the first capacitor and the second capacitor, insuch a way that a first resistor of the voltage-divider bridge isconnected in parallel with the first capacitor.
 5. The electroniccircuit according to claim 4 wherein, where Cc1 and Cc2 are values ofthe first capacitor and of second capacitor respectively, and where R1and R2 are values of the first resistor and of a second resistorrespectively constituting the voltage-divider bridge, the followingrelationship is satisfied: R2×(Cc1+Cc2)=(R1+R2)×Cc1  (8)
 6. Theelectronic circuit according to claim 4 wherein total resistance valueof the voltage-divider bridge is sufficiently high to limit currentconsumption in the voltage-divider bridge to a maximum of a few hundredsof nano-amperes.
 7. The electronic circuit according to claim 4, whereinthe first stage, the second stage, and the circuit for splitting polesare part of an HF7CMOS monolithic integrated circuit, and furthercomprising a power-supply terminal delivering a power-supply voltagehigher than 10 volts.
 8. The electronic circuit according to claim 4,wherein the terminal of the circuit for splitting poles delivering asubstantially constant voltage, is a power-supply terminal of theelectronic circuit.
 9. The electronic circuit according to claim 8wherein the power-supply terminal delivers a power-supply voltage higherthan 10 volts.
 10. The electronic circuit according to claim 4 whereinthe voltage-divider bridge consists of a first resistor and of a secondresistor in series, and wherein at least one of said first and secondresistor is formed by at least one long-channel MOS transistor connectedas a diode.
 11. The electronic circuit according to claim 4 wherein thevoltage-divider bridge consists of a first resistor and of a secondresistor in series, and wherein both of said first and second resistorare formed by at least one long-channel MOS transistor connected as adiode.
 12. The electronic circuit according to claim 4 wherein totalresistance of the voltage-divider bridge is substantially equal to 30megohms, for a power-supply voltage substantially equal to 15 volts. 13.The electronic circuit according to claim 4 wherein, the first stagehaving a differential structure, the electronic circuit furthercomprises a compensation impedance linked to a defined node of the firststage whereby this node exhibits a static potential substantially equalto the static potential of the output of the first stage.
 14. Theelectronic circuit according to claim 13 wherein the compensationimpedance is arranged between said terminal of the circuit for splittingpoles which delivers a substantially constant voltage on the one hand,and said defined node of the first stage on the other hand, and whereinthe compensation impedance exhibits a resistance value which issubstantially equal to the total resistance value of the voltage-dividerbridge of the circuit for splitting poles.
 15. The electronic circuitaccording to claim 4 wherein the circuit for splitting poles inindirectly linked to the first stage.
 16. The electronic circuitaccording to claim 4 wherein the circuit for splitting poles inindirectly linked to the second stage.
 17. An electronic systemcomprising a first stage having a first output; a second invertingvoltage-amplifier stage having a second output; and a circuit forsplitting poles linking the first and second outputs, the circuit forsplitting poles comprising: a first resistor having a first terminalconnected to the first ouput of the first stage and a second terminallinked to a constant voltage supply; a first capacitor having a firstterminal connected to the first terminal of the first resistor and asecond terminal connected to the second terminal of the first resistor;and a second capacitor having a first terminal connected to the secondterminal of the first capacitor and a second terminal being linked tothe second output of the second stage.
 18. The electronic system ofclaim 17, further comprising a second resistor having a first terminalconnected to the second terminal of the first resistor and a secondterminal connected to the constant voltage supply.
 19. The electronicsystem of claim 17, further comprising a third resistor having a firstterminal connected to the second terminal of the second capacitor and asecond terminal connected to the second output of the second stage.